The present invention relates to a unimorph-type ultrasound probe and a method for manufacturing the same. Particularly, the present invention relates to a unimorph-type probe, which is prepared by a micromachining technique, and a method for manufacturing the same.
Conventionally, in the medical field, an ultrasound diagnostic apparatus using ultrasound images has been put to practical use. Generally, in this type of ultrasound diagnostic apparatus, an ultrasonic beam is transmitted toward the inside of a subject from an ultrasound probe, ultrasonic echo from the subject is received by the ultrasound probe, and the received signal is electrically processed, thereby generating an ultrasound image.
As the ultrasound probe, a transducer using a bulk piezoelectric substance has been used. However, in recent years, for example, as disclosed in JP 4043790 B and JP 2008-535643 A, a unimorph-type probe prepared by a micromachining technique has drawn attention because a micro-sized array transducer can be easily manufactured.
The unimorph-type probe is also referred to as a so-called pMUT (Piezoelectric Micromachined Ultrasound Transducer) and has a piezoelectric element which is formed by laminating a lower electrode layer, a piezoelectric substance layer, and an upper electrode layer in this order on a vibration plate formed by partially machining a substrate formed of silicon or the like.
In order to generate a high-accuracy ultrasound image by applying the ultrasound probe to ultrasound diagnosis, the piezoelectric element needs to be driven at a high speed in a MHz band. However, if the piezoelectric element is driven at such a high speed, stress that is exerted between the respective layers, which are laminated on each other and come into close contact with each other, becomes significant. Furthermore, if the ultrasound probe is used over a long period of time, the layers adjacent to each other may be peeled from each other. If the layers are peeled from each other, it is difficult for the ultrasound probe to perform its function. When the sound pressure generated is relatively low, the peeling of layers from each other is not a problem because the workload of the piezoelectric element is small. However, particularly when a high pressure sound is generated, the peeling markedly occurs because the workload of the piezoelectric element is increased.
An acoustic impedance of the unimorph-type probe is generally about 4×106 kg/m2 s to about 10×106 kg/m2 s which is much lower than the acoustic impedance of the conventional probe using a bulk piezoelectric substance formed of an inorganic material. However, the acoustic impedance is higher than the acoustic impedance of a living body as a subject of an ultrasound diagnosis, which is about 1.5×106 kg/m2 s. Accordingly, if the unimorph-type probe is used as is to make an ultrasound diagnosis, transmittance of an ultrasonic beam transmitted to the living body is reduced. Particularly, although the unimorph-type probe is suited for being prepared by microfabrication, it cannot easily realize a high sound pressure and high reception sensitivity. Moreover, the reduction in the transmittance of the ultrasonic beam resulting from the mismatching of the acoustic impedance causes a significant problem in making a highly accurate ultrasound diagnosis.
As described above, hitherto, nothing has been known regarding how the structure of the unimorph-type probe should be designed considering the application thereof to the ultrasound diagnosis, and thus a solution to the problem is required.